In piezoelectric resonator having a thickness vibration-type piezoelectric plate such as an AT-cut crystal plate, generally a pair of driving electrodes are formed directly facing each other on the front and rear surfaces of the piezoelectric plate, and alternating voltage is applied to the driving electrodes. The characteristics of such piezoelectric resonator depend on the structure of their driving electrodes. For example, by using large-sized electrodes, their vibration region can be enlarged, their series resonance resistance can be improved, and their variable frequency range can be broadened. And further, the characteristics of piezoelectric resonator significantly depend on the characteristics of their piezoelectric plate as well. For example, in some cases the planes of their piezoelectric plate are insufficient in plane parallelism due to working conditions or variations in working. In those cases, spurious vibrations are driven intensely, and therefore the characteristics of piezoelectric resonator may be deteriorated.
As for descriptions of the deterioration in their characteristics attributed to the variations in the plane parallelism of the piezoelectric plates, a description is given in JP 2001-196890A (Patent Document 1), for example. In Patent Document 1, a structure is disclosed in which one of the counter electrodes (driving electrodes) consists of two split electrodes, and the two split electrodes roughly match their resonance frequency with that of the other counter electrode to improve the characteristics of the resonator; and the two split electrodes are electrically connected to each other by a conducting means.
The above problem is attributed to spurious vibrations coming to the surface as higher frequencies are used, and piezoelectric plates for high frequency applications in which their driving region is formed thinly are particularly susceptible to such a problem. And further, in voltage-controlled piezoelectric oscillators in which the frequency of their main vibration is varied by changing external voltage, the above problem is brought to the fore when their frequency is varied considerably. There is a high possibility that the variations in the frequency of their main vibration couple with the spurious vibration described above, which has sometimes caused the problems of the jump phenomenon of their frequency occurring and the driving of their vibration becoming unstable.